Sets (Data Structures)

Sets are unordered collections of simple objects. These are used when the existence of an object in a collection is more important than the order or how many times it occurs.

Using sets, you can test for membership, whether it is a subset of another set, find the intersection between two sets, and so on.

>>> bri = set(['brazil', 'russia', 'india'])
>>> 'india' in bri
True
>>> 'usa' in bri
False
>>> bric = bri.copy()
>>> bric.add('china')
>>> bric.issuperset(bri)
True
>>> bri.remove('russia')
>>> bri & bric # OR bri.intersection(bric)
{'brazil', 'india'}

How It Works

If you remember basic set theory mathematics from school, then this example is fairly self-explanatory. But if not, you can google “set theory” and “Venn diagram” to better understand our use of sets in Python.

References

When you create an object and assign it to a variable, the variable only refers to the object and does not represent the object itself! That is, the variable name points to that part of your computer’s memory where the object is stored. This is called binding the name to the object.

Generally, you don’t need to be worried about this, but there is a subtle effect due to references which you need to be aware of:

Example (save as ds_reference.py):

Example:

print('Simple Assignment')
shoplist = ['apple', 'mango', 'carrot', 'banana']
# mylist is just another name pointing to the same object!
mylist = shoplist

# I purchased the first item, so I remove it from the list
del shoplist[0]

print('shoplist is', shoplist)
print('mylist is', mylist)
# Notice that both shoplist and mylist both print
# the same list without the 'apple' confirming that
# they point to the same object

print('Copy by making a full slice')
# Make a copy by doing a full slice
mylist = shoplist[:]
# Remove first item
del mylist[0]

print('shoplist is', shoplist)
print('mylist is', mylist)
# Notice that now the two lists are different

Output:

Simple Assignment
shoplist is ['mango', 'carrot', 'banana']
mylist is ['mango', 'carrot', 'banana']
Copy by making a full slice
shoplist is ['mango', 'carrot', 'banana']
mylist is ['carrot', 'banana']

How It Works

Most of the explanation is available in the comments.

Remember that if you want to make a copy of a list or such kinds of sequences or complex objects (not simple objects such as integers), then you have to use the slicing operation to make a copy. If you just assign the variable name to another name, both of them will ‘’refer’’ to the same object and this could be trouble if you are not careful.

Note for Perl programmers

Remember that an assignment statement for lists does not create a copy. You have to use slicing operation to make a copy of the sequence.